The technology described herein relates generally to turbomachinery, particularly to gas turbine engines, and more particularly, to an acoustic damping apparatus to control dynamic pressure pulses in a gas turbine engine combustor.
Destructive acoustic pressure oscillations or pressure pulses may be generated in combustors of gas turbine engines as a consequence of normal operating conditions depending on fuel-air stoichiometry, total mass flow, and other operating conditions. The current trend in gas turbine combustor design towards low NOx emissions required to meet federal and local air pollution standards has resulted in the use of lean premixed combustion systems in which fuel and air are mixed homogeneously upstream of the flame reaction region. The fuel-air ratio or the equivalence ratio at which these combustion systems operate are much “leaner” compared to more conventional combustors in order to maintain low flame temperatures which in turn limits production of unwanted gaseous NOx emissions to acceptable levels. Although this method of achieving low emissions without the use of water or steam injection is widely used, the combustion instability associated with operation at low equivalence ratio also tends to create unacceptably high dynamic pressure oscillations in the combustor which can result in hardware damage and other operational problems. Pressure pulses can have adverse effects on an engine, including mechanical and thermal fatigue to combustor hardware. The problem of pressure pulses has been found to be of even greater concern in low emissions combustors since a much higher content of air is introduced to the fuel-air mixers in such designs.
Aircraft engine derivative annular combustion systems with their short compact combustor design have been observed to produce complex predominant acoustic pressure oscillation modes in the combustor. The complex modes are characterized as having a circumferential mode coupled with standing axial oscillation modes between the two reflecting surfaces. Each of the two reflecting surfaces is located at an end of the combustor corresponding to compressor outer guide vanes (OGV) and a turbine nozzle inlet. This creates high dynamic pressure oscillations across the entire combustion system.
Several attempts have been made to eliminate, prevent, or diminish the acoustic pressures produced by such dynamic pressure pulses in gas turbine engine combustors. One method has been to elevate flame temperatures, which has achieved moderate success. However, elevating flame temperature is clearly contrary to the goals of low emissions in modern combustors since a relatively low temperature band is preferred. Moreover, it has been found that elevating the flame temperature in a combustor has an undesirable effect on the liners thereof.
Another proposed system has been to utilize an asymmetric compressor discharge pressure bleed. In this system, it is believed that pressure pulses in the combustor take the form of a circumferential pulse located adjacent to the combustion chamber. However, it has been found that pressure pulses within the combustor travel not only in a circumferential manner, but also in an axial manner. More specifically, pulses originating in the combustion chamber travel therein and then are reflected back through the fuel-air mixers into the cold section of the combustor. Therefore, the asymmetric compressor discharge pressure bleed has been found to be unsuccessful in effectively combating pressure pulses in the combustor.
Still another method of counteracting pressure pulses within a gas turbine engine combustor has been the use of detuning tubes positioned at the upstream side of the combustor. These detuning tubes extend into the chamber in front of the combustor by a predetermined amount and are effective at balancing out pressure pulses having a fixed amplitude and frequency. Nevertheless, it has been found that pressure pulses within a combustor are dynamic with changing amplitudes and frequencies. Thus, the aforementioned detuning tubes have met with only a moderate degree of success.
Active acoustic or pressure oscillation control systems have also been suggested to solve the problem. One such idea is disclosed in U.S. Pat. No. 5,575,144, which provides an apparatus for actively controlling dynamic pressure pulses in a gas turbine engine combustor and includes a means for sensing pressure pulses in the combustor, a first processing means for determining the amplitude and frequency for a predominant pressure pulse of the sensed pressure pulses, a second processing means for calculating an amplitude, a frequency, and a phase angle shift for a cancellation pulse to offset the predominant pressure pulse, and an air bleed means for periodically extracting metered volumes of air from the combustor to produce the cancellation pulse, wherein the air bleed means is controlled by the second processing means. Such a system is complex, has many movable parts, that are subject to wear and break down thus requiring repair or replacement. Operators and manufacturers prefer to use less complex methods.
It is highly desirable to have a static means for eliminating or reducing these high dynamic pressure oscillations in a gas turbine engine combustor particularly one that has a short length and is designed for low NOx (nitrous oxides), CO, and unburnt hydrocarbon emissions. It is also highly desirable to develop such an apparatus that can eliminate, prevent, or diminish complex mode acoustic pressure oscillations having different amplitudes and frequencies and that does not have any adverse effect on the emissions of the combustor.